Method for producing soi substrate

ABSTRACT

A method for easily manufacturing a transparent SOI substrate having: a main surface with a silicon film formed thereon; and a rough main surface located on a side opposite to a side where the silicon film is formed. A method for manufacturing transparent SOI substrate, having a silicon film formed on a first main surface of the transparent insulating substrate, while a second main surface of the transparent insulating substrate, an opposite to the first main surface, is roughened. The method includes at least the steps of: roughening the first main surface with an RMS surface roughness lower than 0.7 nm and the second main surface with an RMS surface roughness higher than the surface roughness of the first main surface to prepare the transparent insulating substrate; and forming the silicon film on the first main surface of the transparent insulating substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing an SOIsubstrate, and particularly relates to a method for manufacturing an SOIsubstrate in which a silicon film is formed on one main surface of atransparent insulating substrate.

2. Description of Related Art

In order to achieve higher performance of semiconductor devices,attention has been focused on SOI (silicon on insulator) substrates inrecent years. Additionally, some types of SOI substrates such as asilicon on quartz (SOQ) substrate and a silicon on glass (SOG) substratewhich have support substrates (handle wafers) made of non-siliconmaterials are also expected to be applied to TFT-LCDs, high-frequency(RF) devices, other MEMS products, and so forth (for example, seeJP2006-324530A).

There has been proposed a method for manufacturing the aforementionedSOQ substrate or the like, by using, for example, a silicon substratefor a donor wafer and a quartz substrate for a handle wafer, and bondingthese different substrates together. Since the quartz substrate istransparent in the above bonded substrate, the bonded substrate may havedifferent problems in process and evaluation from the problems of normalSOI substrates manufactured by bonding a silicon substrate to anothersilicon substrate.

One of such problems is that, when being conveyed on equipment, an SOIsubstrate including a silicon film formed on a transparent insulatingsubstrate such as an SOQ substrate (hereinafter, sometimes abbreviatedas a transparent SOI substrate) is unlikely to be recognized by anoptical sensor for recognizing substrates, for example.

Meanwhile, a sandblasting method is used for applying a foggingtreatment (frost treatment) to a substrate, component and so forth basedon SiO₂ such as glass, quartz in some cases. In this method, ato-be-treated surface is blasted with alumina or silica fine particlesto be roughened. The method is widely applied for diverse applications.

However, in the fields of electronic materials and devices, foggysurfaces formed by such a method have several problems. One of them is aparticle (foreign matter) problem. This problem is caused by a sandblastpowder remaining on a treated surface; dust generated from an acuteportion, a crack and a damaged portion of a roughened surface; and soforth. These problems cannot be solved by normal cleaning in many cases.Moreover, a problem such as metal contamination attributable to thisforeign matter is also serious in the field of electronic materials.

This particle problem can be critical particularly when a producttreated by this fogging treatment is used in the field of semiconductor.For example, a quartz boat or the like for a wafer to be used in adiffusion furnace or the like is sometimes subjected to a foggingtreatment to prevent the wafer from closely adhering to a groove forholding the wafer. In this case, since the quartz boat or the likeundergoes a high-temperature process, certain measurement needs to betaken against metal contamination in addition to particles.Additionally, there is a problem that particles on transparentsubstrates such as those of SOQ (Silicon on Quartz) and SOG (Silicon onGlass) significantly increase in amount when back surfaces of thetransparent substrates are subjected to the fogging treatment for makingthe transparent substrates recognizable by substrate-recognition sensorsof various apparatuses.

To remove the particles after such a sandblasting process, a cleaningstep is performed. In this cleaning step, HF cleaning has been employed,for example. The HF cleaning, however, has a problem of making theparticle level rather worse. This is because the HF cleaning activatesthe surface of a glass or the like; moreover, fine pieces of the glassor the like released during the cleaning reattach to the surface (forexample, see Science of Silicon, Chapter 4, Section 4, Realize Science &Engineering Center Co., Ltd.). Furthermore, if the cleaning is performedfor a long period with high concentration HF to remove the particles,the surface subjected to the fogging treatment is made excessivelysmooth, thereby decreasing the effect of roughing the surface.

SUMMARY OF THE INVENTION

The present invention has been made in view of problems as describedabove. An object of the present invention is to provide a method foreasily manufacturing an SOI substrate which comprises a transparentinsulating substrate and a silicon film formed on a main surface of thesubstrate, while the other main surface thereof (an opposite to the mainsurface having the silicon film formed thereon) is roughened.

The present invention has been made to address the above problems. Thepresent invention provides a method for manufacturing an SOI substrate,the SOI substrate comprising at least a transparent insulating substrateand a silicon film, wherein the silicon film is formed on a first mainsurface of the transparent insulating substrate, while a second mainsurface of the transparent insulating substrate, an opposite to thefirst main surface, is roughened. The method comprises at least thesteps of: roughening the first main surface with an RMS surfaceroughness lower than 0.7 nm and the second main surface with an RMSsurface roughness higher than the surface roughness of the first mainsurface to prepare the transparent insulating substrate; and forming thesilicon film on the first main surface of the transparent insulatingsubstrate.

As described above, the method for manufacturing an SOI substratecomprising the steps of roughening and forming makes it possible toeasily manufacture an SOI substrate comprising a transparent insulatingsubstrate and a silicon film formed thereon and having a back surface (amain surface where the silicon film is not formed) roughened.

Moreover, according to the above SOI substrate, the back surface of thetransparent insulating substrate has a high surface roughness, andscatters a signal from a recognition unit containing an optical sensor,thereby successfully preventing a problem that a substrate is notrecognized with the recognition unit. Additionally, this helps toprevent slippage of the substrate during the conveyance, and the like.

According to a method for manufacturing an SOI substrate of the presentinvention, it is possible to easily manufacture an SOI substrate(transparent SOI substrate) comprising a transparent insulatingsubstrate and a silicon film formed thereon and having a back surface (amain surface where the silicon film is not formed) roughened.

Moreover, according to the above SOI substrate, the back surface of thetransparent insulating substrate has a high surface roughness, andscatters a signal from a recognition unit containing an optical sensor,thereby allowing the recognition unit to recognize the substrate.Additionally, this helps to prevent slippage of the substrate during theconveyance, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing one example of a method for manufacturingan SOI substrate according to the present invention.

FIG. 2 is a flowchart showing one example of a specific mode of themethod for manufacturing an SOI substrate according to the presentinvention.

FIG. 3 is a flowchart showing one example of a cleaning step for asurface of a glass substrate subjected to a sandblasting process.

FIG. 4 is a schematic view of a state where a wafer is disposed in acleaning cassette when cleaning is performed to estimate the number offoreign matter adhered to the wafer.

FIG. 5 shows a result of measuring the number of particles.

FIG. 6 shows a result of measuring the number of particles.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in more details.

As described above, conventionally, an SOI substrate comprising atransparent insulating substrate and a silicon film formed thereon, suchas an SOQ substrate, has one of the problems that the SOQ substrate hasdifficulty in being recognized by an optical sensor for recognizingsubstrates while the SOQ substrate is conveyed on equipment, forexample. Furthermore, there is another problem that the amount ofparticles significantly increases when a sandblasting process isperformed on a glass, and then rather increases even though subsequentcleaning is performed.

The present inventors have discovered the following facts against theseproblems. Specifically, prepared in advance is a transparent insulatingsubstrate having main surfaces with different surface roughnesses. Asilicon film is formed on one of the main surfaces, a smooth mainsurface, of this transparent insulating substrate. This enables tomanufacture, easily without complicated steps, an SOI substratecomprising a transparent insulating substrate and a silicon film formedthereon and having a rough back surface (a main surface opposite to themain surface where the silicon film is formed). Such an SOI substrate iscapable of preventing a recognition unit from failing to recognize thesubstrate. Furthermore, the inventors have discovered that cleaning canbe performed effectively in the following manner. Specifically, first,HF cleaning is performed on a sandblasted surface of a glass substrateto etch the source of particles. Then, alkali cleaning is performed inorder that foreign matter released and adhered to the glass substrate inthe course of the HF cleaning can be removed and prevented fromre-adhering. Accordingly, effective cleaning is carried out. Thus, theinventors have completed the present invention.

Hereinbelow, embodiments of the present invention will be described withreference to the drawings. However, the present invention is not limitedthereto.

FIG. 1 is a flowchart showing one example of a method for manufacturingan SOI substrate of the present invention.

The entire flow will be described. A transparent insulating substrate isprepared which has one main surface with a surface roughness higher thanthat of the other main surface (step a). A silicon film is formed on thesmoother one of the main surfaces of the transparent insulatingsubstrate (step b). Thus, an SOI substrate is manufactured which has therough back surface and comprises the silicon film formed on thetransparent insulating substrate.

Specifically, in step a, a transparent insulating substrate 10 isprepared as shown in FIG. 1( a). Note that, among main surfaces of thetransparent insulating substrate 10, a main surface on which a siliconfilm is formed in step b is called a “first main surface” in thisdescription for convenience, while a main surface opposite to the firstmain surface is called a “second main surface.” The transparentinsulating substrate 10 has a first main surface 11 with an RMS surfaceroughness lower than 0.7 nm, and has a second main surface 12 with anRMS surface roughness higher than the surface roughness of the firstmain surface. The reason why the two main surfaces have such surfaceroughnesses will be described later. The RMS is the square root of theaverage of the squares of deviations from an average line to a measuredcurved line.

Incidentally, the kind of the transparent insulating substrate usable inthe present invention is not particularly limited. For example, any of aquartz substrate, a glass substrate, and a sapphire substrate can beused, and can be appropriately selected depending on the purpose or thelike of a semiconductor device to be fabricated after the selectedsubstrate is formed into an SOI substrate.

In this case, the step of roughening preferably comprises at least:performing double-sided lapping and etching on the first main surfaceand the second main surface; and then performing single-sided polishingon only the first main surface.

If the transparent insulating substrate is prepared in the abovedescribed manner by at least performing the double-sided lapping and theetching and then performing the single-sided polishing, there arefollowing advantages. Specifically, a damaged layer after the lappingcan be removed, and generation of particles from the roughened backsurface can be effectively suppressed. Moreover, since only one surfaceneeds to be polished, the cost reduction is achieved in comparison witha case where one surface is roughened after double-sided polishing.

In this case, it is preferable to comprise annealing the transparentinsulating substrate after the double-sided lapping and the etching butbefore the single-sided polishing.

The annealing performed after the double-sided lapping and the etchingas described above can effectively prevent the change in the wafer shapeduring the subsequent single-sided polishing.

Next, in step b, a silicon film 31 is formed on the first main surface11 of the transparent insulating substrate 10 as shown in FIG. 1( b).Thus, an SOI substrate 30 is formed.

In the method for manufacturing an SOI substrate of the presentinvention, the step of forming a silicon film can comprise at least:implanting hydrogen ions and/or rare gas ions from a surface of asilicon substrate or a surface of a silicon substrate having an oxidefilm formed thereon to form an ion-implanted layer; tightly bonding theion-implanted surface of the silicon substrate or the ion-implantedsurface of the silicon substrate having the oxide film formed thereononto the first main surface of the transparent insulating substrate; andpeeling the bonded silicon substrate or the bonded silicon substratehaving the oxide film thereon along the ion implanted layer as aboundary to form the silicon film on the first main surface of thetransparent insulating substrate.

Forming the silicon film by the peeling using the ion implanted layer asa boundary after the ion implantation in the above-described mannerenables forming a highly-crystalline silicon film thinly.

Moreover, the transparent insulating substrate is preferably a quartzsubstrate, a glass substrate, or a sapphire substrate.

The transparent insulating substrate used in the method formanufacturing an SOI substrate of the present invention can beappropriately selected among these depending on the purpose of asemiconductor device to be fabricated.

In step a of roughening, the specific production method for such atransparent insulating substrate 10 is not particularly limited.Additionally, in step b, the formation method for the silicon film 31 onthe first main surface 11 is not particularly limited, either. However,these can be performed as follows, for example.

FIG. 2 shows an example of a more specific mode of the method formanufacturing a transparent insulating substrate according to thepresent invention. Note that FIGS. 2( a-1) to (a-3) correspond to step adescribed above, while FIGS. 2( b-1) to (b-4) correspond to step bdescribed above.

First, as schematically shown in FIG. 2( a-1), a transparent insulatingsubstrate 10′ is prepared which has two main surfaces being roughsurfaces (substep a-1). For example, a quartz substrate cut out from aquartz ingot in the form of slice can be used. In this state, both ofthe main surfaces of the transparent insulating substrate 10′ are roughsurfaces that are relatively unadjusted.

Next, as schematically shown in FIG. 2( a-2), double-sided lapping isperformed in which the two main surfaces of the transparent insulatingsubstrate 10′ are lapped (substep a-2). Note that, in this case, etchingwith hydrofluoric acid or the like is preferably performed so as toremove a damaged layer after the lapping.

Here, the double-sided lapping is preferably performed in such a modethat the two surfaces are treated simultaneously because it is easy toperform. However, the treatment may be performed on one surface at atime.

The two main surfaces of a transparent insulating substrate 10″subjected to the double-sided lapping and the etching as described aboveare rough surfaces having surface roughnesses relatively adjusted.

Next, as schematically shown in FIG. 2( a-3), polishing is performedonly on one surface of the transparent insulating substrate 10″subjected to the double-sided lapping and the etching (substep a-3). Thepolished main surface serves as the first main surface (i.e., thesurface where a silicon film is to be formed later) 11, while theun-polished main surface serves as the second main surface 12.

In this manner, through substeps a-1 to a-3, the transparent insulatingsubstrate 10 can be manufactured which has the first main surface 11with an RMS surface roughness lower than 0.7 nm, and which has thesecond main surface 12 with an RMS surface roughness higher than thesurface roughness of the first main surface.

Incidentally, after the double-sided lapping and the etching (substepa-2), the transparent insulating substrate 10 may be annealed. It ispreferable to perform the annealing after the double-sided lapping andthe etching as described above because the annealing can effectivelyprevents the change in the wafer shape due to the subsequentsingle-sided polishing (substep a-3).

Meanwhile, the example has been shown herein that the transparentinsulating substrate having the two rough main surfaces is prepared insubstep a-1. However, it is only necessary in step a to prepare atransparent insulating substrate which finally has the first mainsurface with an RMS surface roughness lower than 0.7 nm and the secondmain surface with an RMS surface roughness higher than the surfaceroughness of the first main surface. The transparent insulatingsubstrate prepared in substep a-1 does not necessarily have the tworough surfaces. For example, when a transparent insulating substratehaving two main surfaces mirror-polished is prepared, and subsequentlysubjected to double-sided lapping and etching (substep a-2), annealing,and single-sided polishing (substep a-3), the transparent insulatingsubstrate 10 satisfying the above-described surface roughnesses can bemanufactured.

Next, in step b, the silicon film can be formed specifically as follows,for example.

First, as shown in FIG. 2( b-1), a silicon substrate 20 is prepared(substep b-1). Alternatively, a silicon substrate having an oxide filmformed on a surface thereof may be optionally used. To achieve anexcellent bonding state, a surface to be bonded (bonding surface) needsto have flatness at a certain level or higher. For this reason, at leastthe surface to be bonded is subjected to mirror-polishing or the like.This flatness is desirably lower than 0.7 nm in the RMS value, forexample.

Next, as shown in FIG. 2( b-2), hydrogen ions are implanted from thesurface (ion-implanted surface 22) into the silicon substrate 20 to forman ion implanted layer 21 (substep b-2).

It is not limited to the hydrogen ions that are implanted for theformation of this ion implanted layer 21. Instead, rare gas ions or bothhydrogen ions and rare gas ions may be implanted. The implantationenergy, implantation dose, implantation temperature, and other ionimplantation conditions should be appropriately selected so that a filmhaving a predetermined thickness can be obtained. As the specificexamples, the temperature of the substrate during the implantation maybe set to 250 to 400° C., the ion implantation depth may be set to 0.5μm, the implantation energy may be set to 20 to 100 keV, and theimplantation dose may be set to 1×10¹⁶ to 1×10¹⁷/cm². However, the ionimplantation conditions are not limited thereto.

Incidentally, a single crystal silicon substrate having an oxide filmformed on a surface or surfaces thereof can be optionally used. Usingsuch a silicon substrate having the oxide film formed on the surface andimplanting ions through the oxide film achieve an effect of suppressingchanneling of the implanted ions. This can further suppress thevariation in the ion implantation depth. Consequently, a film having arelatively uniform thickness can be formed.

Next, as shown in FIG. 2( b-3), the ion-implanted surface 22 of thesilicon substrate 20 is bonded onto the first main surface 11 of thetransparent insulating substrate 10 (substep b-3).

In bonding the silicon substrate 20 onto the transparent insulatingsubstrate 10,

the first main surface 11 and the ion-implanted surface 22 are thesufficiently flat surfaces as mentioned above. Accordingly, the twosubstrates, for example, a synthetic quartz substrate and a siliconsubstrate, can be bonded by only bonding at room temperature and beingpressurized.

For stronger bonding, however, the bonding is preferably performed asfollows.

Specifically, a surface activation treatment is desirably performed inadvance on both the ion-implanted surface 22 of the silicon substrate 20and the first main surface 11 of the transparent insulating substrate10. The surface activation treatment may be performed only on either theion-implanted surface 22 of the silicon substrate 20 or the first mainsurface 11 of the transparent insulating substrate 10.

In this event, the surface activation treatment may be a plasmatreatment. When the plasma treatment is performed as the surfaceactivation treatment as described above, the treated surface of asubstrate is activated, for example, in such a way that the number of OHgroups on the surface is increased. Accordingly, when the ion-implantedsurface 22 of the silicon substrate 20 is bonded onto the first mainsurface 11 of the transparent insulating substrate 10 in this state, thesubstrates can be tightly bonded together with a hydrogen bond and soforth. Alternatively, an ozone treatment or the like can be performed asthe surface activation treatment, or several types of treatments may beperformed in combination.

The treatment with plasma may be performed as follows. Specifically, asubstrate cleaned by RCA cleaning or the like is placed in a vacuumchamber. After a plasma gas is introduced therein, the substrate isexposed to a high-frequency plasma of preferably approximately 100 W forapproximately 5 to 30 seconds so as to have its surface subjected to theplasma treatment. Examples of the plasma gas usable include: the plasmaof an oxygen gas for treating a single crystal silicon substrate havingan oxide film formed on a surface thereof; and a hydrogen gas, an argongas, a gas mixture of a hydrogen gas and an argon gas, or a gas mixtureof a hydrogen gas and a helium gas for treating a single crystal siliconsubstrate having no oxide film formed on a surface thereof.Alternatively, a nitrogen gas that is an inert gas may be used.

The treatment with ozone may be performed as follows. Specifically, asubstrate cleaned by RCA cleaning or the like is placed in a chamber inwhich an air has been introduced. After a plasma gas such as a nitrogengas or an argon gas is introduced therein, a high-frequency plasma isgenerated to convert oxygen in the air into ozone. Thus, the surface ofthe substrate is subjected to the ozone treatment.

The substrates can be tightly bonded together without a high temperaturetreatment, as far as the substrates are bonded together, for example,under a reduced pressure or a normal pressure at room temperature whilethe surface subjected to the surface activation treatment as describedabove is used as a bonding surface.

Here, after the silicon substrate is bonded onto the transparentinsulating substrate, a heat treatment step of heating the bondedsubstrates at preferably 100 to 300° C. may be performed.

When the silicon substrate is bonded onto the transparent insulatingsubstrate and then the heat treatment at preferably 100 to 300° C. isperformed on the bonded substrates as described above, the bondingstrength between the silicon substrate and the transparent insulatingsubstrate can be increased. Moreover, the heat treatment at the abovedescribed temperature is less likely to cause thermal strain, crack,detachment, and the like due to a difference in thermal expansioncoefficient attributed to the use of different kinds of materials. Theincrease in the bonding strength can make defects less likely to occurin a peeling step.

Next, the peeling step is performed in which the silicon substrate 20 isseparated at the ion implanted layer 21 so as to transform the siliconsubstrate 20 into film. Thus, as shown in FIG. 2( b-4), a silicon film31 is formed (step b-4).

The separation of this silicon substrate (peeling, transformation intofilm) can be performed, for example, through application of a mechanicalexternal force thereto. The mechanical external force is notparticularly limited. Examples thereof include a gas or liquid blow or aphysical impact to a side surface of the ion implanted layer.

Through the steps as described above, an SOI substrate 30 comprising thefilm 31 on the first main surface 11 of the transparent insulatingsubstrate 10 can be manufactured.

Incidentally, it is needless to say that a series of substeps a-1 to a-3and a series of substeps b-1 to b-2 described above are treatments onrespective substrates, and thus these series of substeps may beperformed in reversed order, or may be performed concurrently.

The reasons why the first main surface 11 of the transparent insulatingsubstrate 10 is made to have an RMS surface roughness lower than 0.7 nmin step a in the present invention are as follows. One reason is that ifthe surface roughness is higher than the value (i.e., lower inflatness), the first main surface 11 does not easily allow the siliconfilm to be bonded thereto by means of bonding or the like. Anotherreason is that even if the silicon film is formed thereon, voids or thelike that are a non-bonded portions occur, thereby making it difficultfor the silicon film to favorably retain its crystallinity.

The lower limit value of the RMS surface roughness of the first mainsurface 11 is not particularly limited, and the higher flatness is morepreferable. The improvement of the flatness, however, involves a costproblem, and thus the RMS surface roughness may be practically set toapproximately 0.1 nm or higher.

On the other hand, when the second main surface 12 of the transparentinsulating substrate 10 has the RMS surface roughness higher than thesurface roughness of the first main surface as described above, therecognition unit recognizes the substrate more easily. Althoughdepending on the performance of the recognition unit and otherconditions, the RMS value is preferably, for example, 0.7 nm or higherbecause such an RMS value makes the recognition easy.

Note that the upper limit of the RMS surface roughness of the secondmain surface 12 is not particularly limited. In view of the point thatthe recognition unit recognizes the substrate more easily, the higherthe value is more preferable. However, in consideration of preventingthe generation of particles, it is preferable that the surface roughnessshould not be higher than necessary. The upper limit of the RMS valuemay be approximately 50 nm, for example.

Meanwhile, when the transparent insulating substrate is a glasssubstrate, the step of roughening can comprise at least sandblasting thefirst and the second main surfaces of the glass substrate, and cleaningthe sandblasted surfaces of the glass substrate, the cleaning comprisingat least alkali cleaning after HF cleaning on the sandblasted surfaces.

In this manner, first, HF cleaning is performed on the sandblastedsurfaces of the glass substrate. By the etching effect of the HFsolution on the glass substrate, a portion which can be a particlesource characteristic of the sandblasting process such as an acuteportion, a crack and a damaged portion of the sandblasted glass surfacescan be removed. After the etching, foreign matter released andre-adhered to the surfaces during the HF cleaning can be removed by thealkali cleaning. Thus, the glass substrate is obtainable which has anextremely small number of particles even after sandblasting. Moreover,since the alkali cleaning is performed using an alkali solution for thisremoval of the foreign matter, the foreign matter once removed hardlyre-adheres in the alkali solution, thereby achieving effective cleaning.

In this case, the glass substrate may be a quartz glass substrate.

Even in a case of such an insulator to which foreign matter isespecially likely to adhere, foreign matter released during the HFcleaning after sandblasting can be removed and prevented fromre-adhering, thereby achieving effective cleaning.

Moreover, the glass substrate may be in a wafer form.

According to this cleaning method, a glass wafer especially having aparticle problem can be made into a wafer without particles.

In this case, the wafer may have a single crystal silicon layer stackedthereon.

Even in a case of a wafer having a single crystal silicon layer stackedon the glass wafer, even when foreign matter released from thesandblasted surfaces adheres to the single crystal silicon layer duringthe HF cleaning, this cleaning method allows the foreign matter to beremoved by the subsequent alkali cleaning. Thus, occurrence of particlesfrom the single crystal silicon layer can be prevented.

In the above case, the HF cleaning can be performed while the singlecrystal silicon layer on the wafer is protected with a protection tapeor an organic protection film.

Protecting the single crystal silicon layer during the HF cleaning inthis manner has the following advantages. Specifically, the singlecrystal silicon layer is etched with the HF solution to a lesser extent.Moreover, foreign matter released from the sandblasted surfaces can beprevented from adhering to the single crystal silicon layer during theHF cleaning. Thus, the wafer is obtainable which has the single crystalsilicon layer with a smaller number of particles.

Meanwhile, the alkali solution used in the alkali cleaning may be anyone of NH₄OH, NaOH, KOH, and CsOH, or any one of these with H₂O₂ added.

The alkali solution used in the alkali cleaning can be appropriatelyselected among these. With the further addition of H₂O₂, such an alkalisolution acquires the oxidizing power, and can remove foreign mattermore effectively.

Furthermore, the alkali solution used in the alkali cleaning ispreferably an SC1 solution, containing at least, in a volume compositionratio, 0.5 to 2 of a 29% by weight aqueous NH₄OH solution, 0.01 to 0.5of a 30% by weight aqueous H₂O₂ solution, and 10 of H₂O.

The use of the SC1 solution having such a concentration composition forthe alkali cleaning allows efficient removal of foreign matter adheredto the glass substrate and prevention of the foreign matter fromre-adhering to the glass substrate. Moreover, making the concentrationratio of H₂O₂ lower than that in the normal SC1 solution as describedabove makes it possible to keep the etching effect of the alkaliappropriate.

Furthermore, the alkali solution used in the alkali cleaning may be analkaline organic solvent.

Such an alkaline organic solvent can be also used in the alkalicleaning.

Meanwhile, the HF cleaning step preferably comprises etching thesandblasted surfaces of the glass substrate by 20 nm or more.

Etching the sandblasted surfaces of the glass substrate by 20 nm or morein this manner can let portions of the sandblasted surfaces which can bethe source of particles and include an acute portion, a crack and adamaged portion, be etched to such an extent that no dust occurs in alater step.

FIG. 3 is a flowchart showing one of embodiments from the sandblastingto the cleaning on the glass substrate.

As shown in FIG. 3, first, the sandblasting is performed on the glasssubstrate.

The method of this sandblasting is not particularly limited. Forexample, a surface to be processed can be roughened by applyingparticles of alumina, quartz, or the like to the surface, using the sameapparatus as the conventional one.

Types of glass substrates to which this cleaning method is applicableinclude a SiO₂-based substrate or the like. For example, the method isapplicable to a quartz glass substrate. Even in a case of such aninsulator that is likely to be charged, this cleaning method can preventthe particles from re-adhering during the cleaning after thesandblasting process. Thus, favorable cleaning can be carried out.

Alternatively, the glass substrate may be in a wafer form, or may be aquartz boat which can be used during a heat treatment on a semiconductorwafer, for example.

Furthermore, the method is also applicable to even a wafer having asingle crystal silicon layer stacked thereon. Even in a case of such awafer having a single crystal silicon layer stacked thereon, alkalicleaning in this cleaning method can remove foreign matter released fromthe processed surfaces and adhered to the single crystal silicon layerduring the HF cleaning and can prevent the foreign matter fromre-adhering. Thus, this method can effectively reduce the number ofparticles. Moreover, since the alkali cleaning is performed after the HFcleaning, the particles can be removed without performing the HFcleaning for a long period, and the number of foreign matter adhered tothe single crystal silicon layer during the HF cleaning can be keptsmall. Furthermore, even in a case of a wafer, such as SOQ (Silicon onQuartz), SOG (Silicon on Glass), or the like where the particle problemespecially occurs, this cleaning method can produce a wafer having fewparticles.

Next, as shown in FIG. 3, HF cleaning is performed on the sandblastedsurfaces of the glass substrate. Any one containing hydrofluoric acidcan be used as hydrofluoric acid for this HF cleaning. For example, ahydrofluoric acid solution, an aqueous buffered hydrofluoric acidsolution, and the like can be used. Moreover, the cleaning method is notparticularly limited. The sandblasted glass substrate may be, forexample, immersed, or the sandblasted surfaces may be cleaned throughspin coating.

By firstly performing HF cleaning on the sandblasted surfaces of theglass substrate, an uneven portion such as a crack which can be aparticle source and which is formed as a result of the sandblasting canbe removed by etching. In this event, the sandblasted surfaces of theglass substrate are preferably etched by 20 nm or more by the HFcleaning. Consequently, the particle source can be removed to such adegree that no dust occurs in a later step.

Meanwhile, when the glass substrate to be cleaned by this cleaningmethod is the wafer having a single crystal silicon stacked thereon, itis preferable to perform this HF cleaning while the single crystalsilicon layer is protected with a protection tape or an organicprotection film. This protection may be formed before the HF cleaning,or before the sandblasting. Forming the protection before thesandblasting can efficiently prevent occurrence of particles from thesingle crystal silicon layer.

As the organic protection film, for example, an organic film such as aphotoresist film can be formed. Additionally, the protection tape can bepasted on the organic film or can be pasted directly on the singlecrystal silicon layer.

When the protection film is removed before the alkali cleaning where fewforeign matter adheres, the particles can be removed from the singlecrystal silicon layer by the alkali cleaning. Alternatively, theprotection film can be removed after the alkali cleaning.

Next, as shown in FIG. 3, the alkali cleaning is performed.

By performing the alkali cleaning after the HF cleaning, foreign matteretched, released and re-adhered during the HF cleaning is removed by thealkali cleaning. Moreover, the foreign matter is also prevented fromre-adhering in the alkali solution. Consequently, particles can beefficiently removed.

As the alkali solution used in this cleaning, any one of NH₄OH, NaOH,KOH, and CsOH, any one of these with H₂O₂ added; or an alkaline organicsolvent such as EDP (Ethylene diamine-pyrocatechol-water), TMAH(Tetramethyl ammonium hydroxide), or hydrazine can be used.

Furthermore, as the alkali solution, an SC1 solution containing, in avolume composition ratio, 0.5 to 2 of a 29% by weight aqueous NH₄OHsolution, 0.01 to 0.5 of 30% by weight aqueous H₂O₂ solution, and 10 ofH₂O is preferably used. The SC1 solution with such a concentrationcomposition ratio has the enhanced cleaning effect of the oxidizingpower of H₂O₂, the concentration ratio of H₂O₂ lower than that of anormal SC1 solution, and an appropriate alkalinity. Accordingly, theetching effect can be maintained, and furthermore foreign matter canalso be prevented from re-adhering during the cleaning.

As described above, first, a particle source characteristic of thesandblasted surfaces of the glass substrate, such as a damaged portion,can be removed by the HF cleaning. Then, by performing the alkalicleaning, foreign matter released and re-adhered during the HF cleaningcan be removed while being prevented from further re-adhering.Consequently, effective cleaning can be carried out. When this cleaningmethod is applied to clean a substrate such as a glass substrate onwhich a fogging treatment needs to be performed by a sandblastingprocess intentionally, it is possible to produce a glass product withfew particles while keeping the effect of the fogging treatment.

EXAMPLES

Hereinbelow, the present invention will be more specifically describedwith Examples and Comparative Examples of the present invention.However, the present invention is not limited thereto.

Example 1

According to the method for manufacturing an SOI substrate by using abonding method, as shown in FIG. 2, thirty transparent SOI substrateswere manufactured as follows.

First, a synthetic quartz substrate 10′ having a diameter of 150 mm wasprepared by being cut out directly from a synthetic quartz ingot(substep a-1).

Next, double-sided lapping was performed on both surfaces of thissynthetic quartz substrate 10′ with a double-sided lapping machine, andetching was performed thereon using hydrofluoric acid (substep a-2).Then, the synthetic quartz substrate was annealed under a non-oxidizingatmosphere at 1100° C. for 30 minutes.

The synthetic quartz substrate 10″ had only one of the surfaces polishedby using a one surface polisher. Thus, one main surface (first mainsurface) 11 had an RMS surface roughness of 0.2 nm (substep a-3). Theother main surface (second main surface) 12 had an RMS surface roughnessof 1.0 nm.

Next, a single crystal silicon substrate having been mirror-polished andhaving a diameter of 150 mm was prepared as a silicon substrate 20.Then, a silicon oxide film layer of 100 nm was formed on surfaces of thesilicon substrate by thermal oxidation (substep b-1).

Next, hydrogen ions were implanted into the silicon substrate 20 throughthe silicon oxide film layer formed thereon to thereby form a microbubble layer (ion implanted layer) 21 which was parallel to the surfacewith respect to the average travel depth of the ions (substep b-2). Asthe ion implantation conditions, the implantation energy was 35 keV, theimplantation dose was 9×10¹⁶/cm², and the implantation depth was 0.3 μm.

Next, the ion-implanted silicon substrate 20 was placed in a plasmatreatment apparatus. After nitrogen gas was introduced as a plasma gastherein, a high-frequency plasma treatment was performed on theion-implanted surface for 10 seconds by applying a high frequency of13.56 MHz between parallel plate electrodes each having a diameter of300 mm, under a high-frequency power of 50 W and a reduced pressure of 2Torr (270 Pa). In this manner, a surface activation treatment wasperformed on the ion-implanted surface 22 of the silicon substrate 20.

Meanwhile, the synthetic quartz substrate 10 was placed in a plasmatreatment apparatus. After nitrogen gas was introduced therein as aplasma gas in a narrow space between electrodes, the synthetic quartzsubstrate 10 was subjected to a high-frequency plasma treatment for 10seconds by applying a high frequency between the electrodes to generateplasma. In this manner, a surface activation treatment was alsoperformed on the first main surface 11 of the synthetic quartz substrate10.

The silicon substrate 20 and the synthetic quartz substrate 10 subjectedto the surface activation treatments as described above were bondedtogether at room temperature, using the activated surfaces as thebonding surfaces. Then, the back surfaces of the respective substrateswere strongly pressed against each other in a thickness direction (stepb-3).

Next, in order to increase the bonding strength, substrates obtained bybonding the silicon substrate 20 and the synthetic quartz substrate 10were subjected to a heat treatment at 300° C. for 30 minutes.

Next, an external impact was applied to the ion implanted layer 21 ofthe silicon substrate 20. The silicon substrate was gradually separatedalong the ion implanted layer 21, and thereby a silicon film 31 was leftbehind (substep b-4).

A transparent SOI substrate 30 comprising the synthetic quartz substrate10 and the silicon film 31 thereon was manufactured in this manner. Arecognition test was conducted on manufactured transparent SOIsubstrates 30 by using a substrate-recognition unit equipped to adevice-fabrication apparatus. As a result, all the substrates wereaccurately recognized.

Example 2, Comparative Examples 1 to 3

First, eight quartz wafers were sandblasted.

Next, each two of the wafers were treated under each of the followingconditions.

Example 2: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in an alkali solution(NH₄OH:H₂O₂:H₂O₂=1:0.2:10 at weight ratio) for 10 minutes.

Comparative Example 1: no cleaning was performed on the wafers.

Comparative Example 2: the wafers were immersed in 2% by weight HFsolution for 30 minutes.

Comparative Example 3: the wafers were immersed in an alkali solution(NH₄OH:H₂O₂:H₂O=1:0.2:10 at weight ratio) for 10 minutes.

Next, four wafers in total were disposed in a cleaning cassette 43 asshown in FIG. 4. Specifically, the four wafers include: two quartzwafers 40 treated under the same conditions as the aforementionedconditions for the quartz wafers 40; and two test silicon wafers 42 notsubjected to the sandblasting. The wafers were disposed in a way thatsandblasted surfaces 41 respectively faced to particle-test surfaces 44of the test silicon wafers 42. Here, the distance between each pair ofthe quartz wafer 40 and the test silicon wafer 42 was set to 5 mm. Inthis state, normal RCA cleaning was performed. Then, the number ofparticles (0.2 μm or larger) on the test surface 44 of the silicon wafer42 was measured with a particle counter. Based on the number ofparticles, the number of foreign matter adhered to the quartz wafer 40was estimated. FIG. 5 shows the measurement result.

As shown in FIG. 5, an extremely small number of particles were measuredfrom the wafers of Example 2 where the HF cleaning was performed andfollowed by the alkali cleaning, in comparison with those in ComparativeExamples 1 to 3. Additionally, a larger number of particles weremeasured from the wafers of Comparative Example 2 where only the HFcleaning was performed, than that in Comparative Example 1 where nocleaning was performed. In comparison with those in Comparative Examples1 and 2, the number of particles was relatively small in ComparativeExample 3 where only the alkali cleaning was performed; however, theremoval of the particle source was unsuccessful.

Examples 3 to 7

First, six quartz wafers were sandblasted.

Next, each two of the wafers were treated under each of the followingconditions.

Example 3: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in 3% by volume NH₄OH solution for 10minutes.

Example 4: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in an alkali solution(NH₄OH:H₂O₂:H₂O=1:1:10 at weight ratio) for 10 minutes.

Example 5: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in an alkali solution(NH₄OH:H₂O₂:H₂O=1:0.2:10 at weight ratio) for 10 minutes.

Example 6: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in an alkaline organic solvent (8%TMAH solution) for 10 minutes.

Example 7: the wafers were immersed in 2% by weight HF solution for 30minutes, and subsequently immersed in an alkali solution (10 wt% KOHsolution) for 10 minutes.

Next, four wafers in total were disposed in a cleaning cassette 43 asshown in FIG. 4. Specifically, the four wafers include: two quartzwafers 40 treated under the same conditions as the aforementionedconditions for the quartz wafers 40; and two test silicon wafers 42 notsubjected to the sandblasting. The wafers were disposed in a way thatsandblasted surfaces 41 respectively faced to particle-test surfaces 44of the test silicon wafers 42. Here, the distance between each pair ofthe quartz wafer 40 and the test silicon wafer 42 was set to 5 mm. Inthis state, normal RCA cleaning was performed. Then, the number ofparticles (0.2 μm or larger) on the test surface 44 of the test siliconwafer 42 was measured with a particle counter. Based on the number ofparticles, the number of foreign matter adhered to the quartz wafer 40was estimated. FIG. 6 shows the measurement result of Examples 3 to 7 inaddition to the measurement result of Comparative Example 2 where onlythe HF cleaning was performed in the same condition as above.

As shown in FIG. 6, the number of particles measured in Examples 3 to 7where the alkali cleaning was performed after the HF cleaning wassignificantly small, in comparison with the number measured inComparative Example 2 where only the HF cleaning was performed in thesame condition as above. Consequently, the number of particles can begreatly reduced by the alkali cleaning. Additionally, the number ofparticles was the smallest by the alkali cleaning in Example 5. This isattributable to a synergy effect of the oxidizing power of H₂O₂ withNH₄OH in the alkali solution of Example 5. Since the alkali solution ofExample 5 has the concentration of H₂O₂ lower than that in the alkalisolution of Example 4, the etching effect of the alkali is notdeteriorated. Consequently, particles are efficiently removed.]

As described above, a cleaning method in which alkali cleaning isperformed after HF cleaning has the following effects. Specifically,unevenness, which can be a particle source characteristic of sandblastedsurfaces of a glass is etched by the HF cleaning. Furthermore, foreignmatter released and adhered during the HF cleaning can be removed by thealkali cleaning while being prevented from re-adhering. Thus, the numberof particles can be efficiently reduced.

It should be noted that the present invention is not limited to theabove-described embodiments. The embodiments are exemplary, andembodiments having substantially the same configuration as andexhibiting similar function and effects to those of a technological ideadescribed in scope of claims of the present application are contained bythe technological scope of the present invention.

1. A method for manufacturing an SOI substrate, the substrate comprisingat least a transparent insulating substrate and a silicon film formed ona first main surface of the transparent insulating substrate, while asecond main surface of the transparent insulating substrate, an oppositeto the first main surface, being roughened, the method comprising atleast the steps of: roughening the first main surface with an RMSsurface roughness lower than 0.7 nm and the second main surface with anRMS surface roughness higher than the surface roughness of the firstmain surface to prepare the transparent insulating substrate; andforming the silicon film on the first main surface of the transparentinsulating substrate.
 2. The method for manufacturing an SOI substrateaccording to claim 1, wherein the step of roughening comprises at least:performing double-sided lapping and etching on the first main surfaceand the second main surface; and then performing single-sided polishingon only the first main surface.
 3. The method for manufacturing an SOIsubstrate according to claim 2, comprising annealing the transparentinsulating substrate after performing the double-sided lapping and theetching but before performing the single-sided polishing.
 4. The methodfor manufacturing an SOI substrate according to claim 1, wherein thestep of forming the silicon film on the first main surface of thesubstrate comprises at least: implanting hydrogen ions and/or rare gasions from a surface of a silicon substrate or a silicon substrate havingan oxide film formed thereon to form an ion implanted layer; bonding theion-implanted surface of the silicon substrate or the silicon substratehaving the oxide film formed thereon onto the first main surface of thetransparent insulating substrate; and peeling the bonded siliconsubstrate or the bonded silicon substrate having the oxide film formedthereon along the ion implanted layer as a boundary to form the siliconfilm on the first main surface of the transparent insulating substrate.5. The method for manufacturing an SOI substrate according to claim 1,wherein the transparent insulating substrate is a quartz substrate, aglass substrate, or a sapphire substrate.
 6. The method formanufacturing an SOI substrate according to claim 1, wherein thetransparent insulating substrate is a glass substrate, the step ofroughening comprises at least sandblasting the first and the second mainsurfaces of the glass substrate, and cleaning the sandblasted surfacesof the glass substrate, and the cleaning comprises alkali cleaning afterHF cleaning on the sandblasted surfaces.
 7. The method for manufacturingan SOI substrate according to claim 6, wherein the glass substrate is aquartz glass substrate.
 8. The method for manufacturing an SOI substrateaccording to claim 6, wherein an alkali solution used in the alkalicleaning is any one of NH₄OH, NaOH, KOH and CsOH, or any one of thesewith H₂O₂ added.
 9. The method for manufacturing an SOI substrateaccording to claim 6, wherein an alkali solution used in the alkalicleaning is an SC1 solution which contains at least, in a volumecomposition ratio, 0.5 to 2 of a 29% by weight aqueous NH₄OH solution,0.01 to 0.5 of a 30% by weight aqueous H₂O₂ solution, and 10 of H₂O. 10.The method for manufacturing an SOI substrate according to claim 6,wherein an alkali solution used in the alkali cleaning is an alkalineorganic solvent.
 11. The method for manufacturing an SOI substrateaccording to claim 6, wherein the HF cleaning comprises etching thesandblasted surfaces of the glass substrate by 20 nm or more.